1. Field of the Invention
The present invention relates to an image forming apparatus utilizing electrophotographic techniques or electrostatic recording techniques, such as a copying machine, a facsimile and a laser printer provided with a heating type fuser.
2. Description of the Related Art
It has been customary in an image forming apparatus utilizing electrophotographic techniques, such as a laser printer, that commands related to printing and coded character and pictorial image information are received from external image processing equipment such as a computer, and when the coded information is converted into image information by a formatter or the like, pictorial images having density information, such as photographs, are binary-coded through known image processing, e.g., the dither matrix process or the error diffusion process, for conversion into the image information. After that, the image information is printed in an electrophotographic engine.
In one of the known electrophotographic engines, an electrostatic latent image is formed by writing an electrostatic image, based on the image information, on an electrophotographic photosensitive member or an electrostatic recording dielectric member which is uniformly charged beforehand; e.g., optically scanning over a photosensitive surface by a semiconductor laser. Also, one of the known developing devices for developing the electrostatic latent image performs development in such a manner, for example, that in a developing region where an electrostatic latent image carrier and a developer carrier are positioned to face each other with a small gap therebetween, a developer is moved from the developer carrier onto the surface of the electrostatic latent image carrier to form a developer (toner) image corresponding to the electrostatic latent image. The toner image thus developed is transferred onto a sheet of recording paper (material) by transfer means.
Subsequently, the recording paper onto which the toner image has been transferred is separated from the electrostatic latent image carrier and sent to fusing means, e.g., a known heating type fuser, where the toner image is fused and fixed to the recording paper.
As heating type fusers, hot-roller fusers have been widely used in the past, and film heating fusers have been used recently. In film heating fusers, particularly, no power is supplied to the fusers in the standby state and power consumption is held as low as possible. More specifically, a heat-fusing method using the film heating technique that a toner image is fused and fixed to a sheet of recording paper through a film interposed between a heater and a press roller, is proposed in Japanese Patent Laid-Open No. 63-313182, No. 2-157878, No. 4-44075, No. 4-204980, etc.
FIG. 7 schematically shows the principal part of a film heating fuser as one example of such heating type fusers. In FIG. 7, the fuser comprises a heating member (referred to as a heater hereinafter) 101 fixedly supported by a stay holder (support) 102, and an elastic press roller 110 held in pressure contact with the heater 101 while a fusing nip portion N is formed in a predetermined nip width, with a heat-resistant, thin film (referred to as a fusing film) 103 sandwiched therebetween. The heater 101 is heated and adjusted to a predetermined temperature upon supply of electric power. The fusing film 103 is a member in the form of a cylinder, an endless belt or a rolled web having ends, and is moved in the direction of an arrow a by the rotating force of not-shown driving means or the press roller 110 while closely contacting with and sliding over the surface of the heater 101 in the fusing nip portion N.
Under the condition where the heater 101 is heated and adjusted to the predetermined temperature and the fusing film 103 is moved in the direction of the arrow, when a sheet of recording paper P having a not-yet-fused toner image t formed and carried thereon as an object to be heated is introduced between the fusing film 103 and the press roller 110 in the fusing nip portion N, the recording paper P is brought into close contact with the surface of the fusing film 103 and transported through the fusing nip portion N along with the fusing film 103 while being held in sandwiched relation. In the fusing nip portion N, the recording paper P and the toner image t are heated by the heater 101 through the fusing film 103 so that the toner image t on the recording paper P is fused and fixed under heating. A part of the recording paper P having passed the fusing nip portion N is peeled off from the surface of the fusing film 103 and then further transported.
FIG. 8 shows the background art of the present invention and is a partial view of an image forming apparatus provided with the heating type fuser mentioned above.
In the image forming apparatus provided with the heating type fuser mentioned above, as the printing speed increases, ejected sheets of recording paper are stacked one above another in a paper ejection tray under the condition where the paper temperature still remains high and the charged static electricity is not yet attenuated.
Particularly, in the F/U (faceup) paper ejection mode in which the sheets of recording paper P are each ejected and stacked with the surface including the toner image t (i.e., the printed surface) facing up, as shown in FIG. 8A, the above tendency is more remarkable because the feed path of the recording paper is shorter and the recording paper is stacked more immediately after being fed out of the fuser than in the F/D (facedown) paper ejection mode in which the sheets of recording paper P are each ejected and stacked with the printed surface facing down, as shown in FIG. 8B. Therefore, when sheets of recording paper having relatively small thermal capacity and high volume resistivity, such as OHP sheets, are fed in the F/U paper ejection mode, the sheets of recording paper ejected to a paper ejection tray 130 still remain at such a high temperature that the sheets lack firmness and the toner is not solidified sufficiently, and still have the charged static electricity not yet attenuated. This may result in the OHP sheets not being stacked one above another satisfactorily because the adhesive force of the toner, the force of returned static electricity, etc. cause the sheets to stick to each other or curl.
Further, in the F/U paper ejection mode, because the sheets of recording paper are generally stacked on the tray 130 which is disposed downstream of a fuser 100 in the direction of pass of the recording paper and is opened as shown in FIG. 8A, paper ejection members 131 (131a, 131b) (such as paper ejection guides) of the fuser are exposed to the exterior such that the user may easily touch them. Nevertheless the paper ejection members are often heated excessively; hence the user feels unpleasant upon touching those members in such a condition.